Method of fabricating semiconductor devices on a group iv substrate with controlled interface properties and diffusion tails

ABSTRACT

Electronic and opto-electronic devices having epitaxially-deposited III/V compounds on vicinal group IV substrates and method for making same. The devices include an AlAs nucleating layer on a Ge substrate. The group IV substrate contains a p-n junction whose change of characteristics during epitaxial growth of As-containing layers is minimized by the AlAs nucleating layer. The AlAs nucleating layer provides improved morphology of the devices and a means to control the position of a p-n junction near the surface of the group IV substrate through diffusion of As and/or P and near the bottom of the III/V structure through minimized diffusion of the group IV element.

This application claims the benefit of priority of U.S. ProvisionalPatent Application No. 60/822,138 filed Aug. 11, 2006, which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to the epitaxial deposition ofelectronic and opto-electronic devices. More particularly, the presentinvention relates to the deposition of III/V electronic andopto-electronic device structures on group IV substrates.

BACKGROUND OF THE INVENTION

The deposition of layer sequences for III/V opto/electronic devices,such as multi-junction solar cells and light-emitting diodes (LEDs), ongroup IV substrates is known. The electronic and optical properties ofsuch devices are being studied extensively and the correlation betweenthese properties and the characteristics of the substrate-epilayerinterface is receiving great attention. The reason for the attentiongiven to the substrate-epilayer interface is that, for the most part,the performance of these devices is determined by the quality of thisinterface.

When depositing a III/V material, for example GaAs, epitaxially on agroup IV substrate, for example Ge, the formation of the appropriateatomic layer sequence of the group III and group V layers is not readilyestablished. The group IV sites (Ge atoms) can bond either group III orgroup V atoms. In practice, some areas of the group IV substrate willbond group III atoms and some other areas will bond group V atoms. Theboundary regions between these different growth areas give rise toconsiderable structural defects, such as anti-phase domains, whichadversely affect the performance of the device.

To curtail some of these undesired events, the group IV substrates areusually vicinal substrates with an off-cut angle ranging from 0-15°.These vicinal substrates offer terraces and step edges where the atomscan attach with different bond configurations, thus providing greaterorder in the growth process.

In devices such as, for example, solar cells having III/V compoundsepitaxially deposited on a group IV substrate, it is often desirable tocreate part of the device itself in the group IV substrate by diffusing,for example, a group V species in the group IV substrate. As an example,for solar cells, if a group V element is diffused in a p-type Gesubstrate, an n-type region is formed, giving rise to a p-n junction.This p-n junction becomes photo-active and can be part of a single ormultijunction solar cell. However, when depositing the III/V compound attypical process temperatures (500-750° C.) on the Ge substrate, thegroup V element of the compound tends to diffuse, with little control,in the substrate thereby making the formation of a predictable p-njunction difficult. In cases involving Ge substrates with a pre-existingp-n junction, as could be the case in the hetero-integration of III-Vopto/electronics on Ge, SiGe and SiC electronic circuits, the depositionof an overlaying III/V compound can modify the doping profile of thepre-existing p-n junction resulting in subpar performance of the p-njunction and device. Consequently, the electrical characteristics arenot easily controllable. In such situations, it can become quitedifficult, if not impossible, to attain and maintain the desired dopingprofile and the electrical characteristics of the substrate's p-njunction, such electrical characteristics including, in the case ofsolar cells, the open circuit voltage (Voc). Furthermore, group IV atomswill diffuse from the substrate into the epitaxially deposited III/Vlayers. Hence, layers within the initial 0.5-1 μm of the III/V layersequence can be highly doped with the group IV element when theexcessive diffusion of group IV atoms is not curtailed through the useof suitable nucleation conditions and materials. Group IV atoms like Siand Ge are, at moderate concentrations, typically n-type dopants inIII/V semiconductor material. However, due to their amphoteric naturethese atoms can cause a large degree of compensation (combinedincorporation of n- and p-type impurities) when incorporated atconcentrations much larger than 2×10¹⁸ cm⁻³, often leading to a strongdeterioration of electrical and optical properties of the hostsemiconductor layer.

U.S. Pat. No. 6,380,601 B1 to Ermer et al., hereinafter referred to asErmer, teaches deposition of GaInP on an n-doped interface layer on ap-type Ge substrate and subsequent deposition of a GaAs binary compoundon the GaInP layer. The phosphorous of the GaInP layer tends to notdiffuse in the Ge substrate as deeply as the arsenic of a GaAs layerwould. Thus, the phosphorous doping and subsequent deposition of theGaInP layer allows better control of the doping profile of the n-typelayer of the Ge substrate and consequently, leads to a better control ofthe electrical characteristics of the p-n junction formed in the Gesubstrate. However, the problem with having a GaInP interfacial layer atthe Ge substrate interface is that the morphology of devices preparedunder typical epitaxial process conditions for these materials is notideal: defects often abound. It would appear that extreme nucleationconditions (temperature, deposition rate, group V overpressure) of theGaInP interfacial layer are required in order to obtain devices withsuitable morphology.

It is, therefore, desirable to provide a method for fabricatingsemiconductor devices having a III/V compound epitaxially deposited on agroup IV substrate under typical epitaxial process conditions, thedevices having a suitable morphology and the method allowing bettercontrol over the optical and electrical interface properties as well asthe diffusion layer in the group IV substrate.

SUMMARY OF THE INVENTION

It is an object of the present invention to obviate or mitigate at leastone disadvantage of previous devices having epitaxial III/V layers on agroup IV substrate.

In a first aspect, the present invention provides a semiconductor devicecomprising a group IV layer; and a nucleating layer formed on the groupIV layer. The nucleating layer includes a Ill-V compound having at leastaluminum (Al) as a group III element and at least one of arsenic (As),nitrogen (N) and antimony (Sb) as a group V element.

In a second aspect, the present invention provides a method offabricating a semiconductor structure on a group IV layer. The methodcomprises a step of forming a nucleating layer on the group IV layer,the nucleating layer including a III-V compound having at least aluminum(Al) as a group III element and at least one of arsenic (As), nitrogen(N) and antimony (Sb) as a group V element. The method further comprisesa step of forming a first III-V compound layer on the nucleating layer.

In a third aspect, the present invention provides a method ofcontrolling the doping profile of a p-n junction formed in a group IVsubstrate. The method comprises a step of forming a nucleating layer onthe group IV substrate, the nucleating layer including a III-V compoundhaving at least aluminum (Al) as a group III element and at least one ofarsenic (As), nitrogen (N) and antimony (Sb) as a group V element. Themethod further comprises a step of forming a III-V compound layer on thenucleating layer, the nucleating layer for controlling the diffusion ofgroup V elements into the group IV substrate and for controlling thediffusion of group IV elements out of the group IV substrate.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the attached Figures, wherein:

FIG. 1 is a side view of an embodiment of the present invention;

FIGS. 2A and 2B are photographs of embodiments of the present inventionwith different thickness of an AlAs nucleating layer;

FIG. 3 is a haze measurement of the embodiment of FIG. 1 as a functionof thickness of the AlAs nucleating layer;

FIG. 4 is a depiction of the deposition process of AlAs on a vicinal Gesubstrate;

FIG. 5 is a graph of SIMS data for different atomic species of thestructure of the embodiment of FIG. 1 when the thickness of the AlAslayer is nil;

FIG. 6 is a graph of SIMS data for different atomic species of thestructure of the embodiment of FIG. 1 when the thickness of the AlAslayer is 11.6 Å;

FIG. 7 is a flow chart of a method of the present invention;

FIG. 8 is a graph of SIMS data for Phosphorous as a function of thethickness of AlAs for the structure of the embodiment of FIG. 1;

FIG. 9 is a graph of SIMS data for Arsenic as a function of thethickness of AlAs for the structure of the embodiment of FIG. 1;

FIG. 10 is a graph of SIMS data for Ge as a function of the thickness ofAlAs for the structure of the embodiment of FIG. 1;

FIG. 11 shows the concentration of Phosphorous as a function sampledepth for four different thicknesses of AlAs for the structure of theembodiment of FIG. 1;

FIG. 12 shows the concentration of Arsenic as a function sample depthfor four different thicknesses of AlAs for the structure of theembodiment of FIG. 1;

FIG. 13 shows the concentration of Ge as a function sample depth forfour different thicknesses of AlAs for the structure of the embodimentof FIG. 1;

FIG. 14 is a current versus voltage plot of a photovoltaic cell having astructure similar to that shown in the embodiment of FIG. 1; and

FIG. 15 is a series of current versus voltage plots for photovoltaiccells fabricated without an AlAs nucleation layer and photovoltaic cellsfabricated with an AlAs nucleation layer.

DETAILED DESCRIPTION

Generally, the present invention provides a method for fabricatingelectronic or opto-electronic devices having a group IV substrate onwhich a III/V layer structure is deposited. The method allows for themanufacturing of devices with improved morphology and controlled dopingprofiles of group V constituents into the group IV substrate and groupIV constituents into the III/V layers.

FIG. 1 shows an exemplary three junction semiconductor structure 18embodying the present invention. Such a structure can be used inmulti-junction solar cells, e.g., three junction solar cells. Further,as will be readily understood by a worker skilled in the art, similarstructures can be used in light-emitting diodes (LEDs) and otherelectronic and/or opto-electronic devices. An AlAs layer 22 of thicknesst₁ is deposited atop a vicinal Ge substrate 20. As will be understood bya worker having ordinary skill in the art, the term “vicinal” hererefers to a crystal plane that is oriented near to a fundamental plane.The angle of the vicinal Ge substrate can range from 0°-20°; the crystalorientation of the Ge substrate can be, for example, 6° towards thenearest <111> plane or any other suitable orientation. On top of theAlAs layer 22 are a GaInP layer 24 having a thickness t₂ and a GaAslayer 26 having a thickness t₃. The deposition of the AlAs layer 22, theGaInP layer 24 and the GaAs layer 26 can be achieved through anysuitable means such as: metal organic chemical vapor deposition (MOCVD),chemical beam epitaxy (CBE), molecular beam epitaxy (MBE), solid phaseepitaxy (SPE), hydride vapour phase epitaxy or by other similar hybridsystems or combinations thereof. Although a Ge substrate 20 is shown,any other suitable group IV substrates, such as, e.g., Si, SiGe or SiCsubstrates, can also be used. Further, as will be understood by theskilled worker, the above also applies in cases where, instead of agroup IV substrate, a device requiring transition from a group IVmaterial to a III-V compound is used. Similarly, the AlAs layer can besubstituted, without departing from the scope of the present invention,with other III-V compound semiconductor alloys with a high concentrationof Al such as, for example, AlN, AlSb or Al(Ga)As.

In FIGS. 2A and 2B, the morphology of the structure 18 is compared fortwo different thicknesses t₁ of the AlAs layer 22. In FIGS. 2A and 2B, atest structure 28 corresponds to the structure 18 with t₁=0 and a teststructure 30 corresponds to test structure 18 with t₁=4 monolayers ofAlAs. FIGS. 2A and 2B show microscope photographs of the top surface oftests structures 28 and 30 where in each case t₂=0.025 μm and t₃=0.2 μm.The test structures 28 and 30 were fabricated by MOCVD at temperaturesranging from 650-730° C. with the deposition rates of GaAs, GaInP andAlAs being respectively 4 μm/hr, 0.8 μm/hr and 0.7-0.42 μm/hr.

As seen in FIG. 2A (GaInP on Ge), the number of defects, shown as whitespeckles, is much higher than if FIG. 2B (AlAs on Ge). The density ofdefects is of the order of thousands per cm in FIG. 2A and essentially 0in FIG. 2B. This type of defect is entirely absent on FIG. 2B. The largespeckle in the central region of FIG. 2B it is attributed to a foreignparticle on the test structure 30, which is not inherent to thenucleation process.

The graph of FIG. 3 shows a plot of haze for the structures 18 as afunction of t₁, the thickness of the AlAs layer 22. The measurement wasperformed with a Surfscan™ haze measurement apparatus manufactured byKLA-Tencor of California. It is very clear from the haze plot thatadding just a fraction of a monolayer of AlAs greatly improves thesurface morphology of the structure 18.

The reason for this improvement in the morphology of III/V compoundsdeposited on vicinal Ge substrates with an intermediate AlAs layer 22between the subsequent III/V compounds is attributable to the following.As shown in FIGS. 4A and 4B, Al atoms are relatively small with respectto As atoms. As such, the Al atoms have an electrochemical potentialthat favors their positioning at the steps 40 present on the vicinal Gesubstrate 20. Thus, introducing Al and As in the growth chamber andallowing sufficient time to pass will see the steps 40 predominantlyoccupied by Al atoms provided that the surface energy is high enough toallow surface reconfigurations due to the substrate temperature. Thisallows for the establishment of a homogeneous growth sequence, whichleads to morphologically sound samples as shown in FIG. 2B in which thenucleation sequence as been properly established and therefore, theanti-phase domain defects have been greatly reduced. This process isknown as a nucleation process and, in the case depicted in FIGS. 4A and4B, can occur at temperatures typical in depositing AlAs layer epitaxiallayers (e.g., 650-730° C.).

FIG. 5 shows a secondary ion mass spectroscopy (SIMS) measurementperformed on a test structure similar to that of the test structure 28of FIG. 2A, i.e., the structure 18 with t₁=0. The line 50 indicates theboundary between the Ge substrate 20 and the III/V compound. As seen inthe SIMS plots of FIG. 5, atomic masses 72 (Ge), 75 (As), 31 (P), 27(Al), 69 (Ga) and 115 (In) are measured as a function of exposure timeto a beam of Cs atoms accelerated by a 3 kV voltage. A depth scalerelating the exposure time to the depth probed by the SIMS beam isshown. Of note is that the germanium isotope 72 is measured instead ofthe prevalent germanium 74. This is done in order to avoid anyinterference with the measurement of As, which has an atomic mass of 75.

As indicated by region 52 of the graph, the diffusion of P occurs intothe Ge substrate and dominates all other species diffusion. This leadsto high levels of n-type conductivity in the Ge substrate, which are notalways desirable. The presence of such levels of P in the Ge substratecan lead to low reverse breakdown voltages, which are not tolerable. Insuch structures, the diffusion of P in the Ge substrate can only becontrolled through temperature and thickness (growth time) of the GaInPnucleation layer on the Ge substrate. This makes for a very difficultcontrol of the parameters of the p-n junction in the Ge substrate.

Consequently, structures such as shown in FIG. 2A where t₁=0, i.e.,structures having GaInP deposited directly on a Ge substrate at atemperature ranging from 650-730° C. at a growth rate of 0.8 μm/hr, notonly exhibit poor morphological qualities but also have an essentiallyuncontrollable n-type doping deep into the Ge substrate. In cases wherethe doping profile is acceptable, the poor morphology of resultingdevices will typically result in lower opto-electronic performance.

FIG. 6 shows SIMS measurements performed on the test structure 30 ofFIG. 2B, i.e., the sample having t₁=4 monolayers (of AlAs) on top the Gesubstrate 20. The line 50 indicates the boundary between the Gesubstrate 20 and the III/V compound. As seen in the SIMS plots of FIG.6, atomic masses 72 (Ge), 75 (As), 31+31+31 (triple ion P), 69 (Ga) and115 (In) are measured as a function of exposure time to a beam of Csatoms accelerated by a 3 kV voltage.

Clearly, the diffusion of P in the Ge substrate is much smaller thanthat shown in FIG. 5. The diffusion depth of P in the Ge substrate isapproximately 0.02 μm and the diffusion of As in the Ge substrate isapproximately 0.10 μm. Thus, when fabricating structures similar to thestructure 18 for solar cells, LEDs or other opto-electronic orelectronic devices, it is much easier to control the doping profile inthe Ge substrate when a high-Al containing alloy such as AlAs is usedfor the nucleating layer.

FIG. 7 depicts processing steps for structures such as the structure 18of FIG. 1. At step 60, a nucleating layer containing AlAs is formed on ap-type group IV substrate. At step 62, epitaxial deposition of a III/Vlayer containing phosphorous is performed together with the formation ofa p-n junction near the surface of the substrate. This is followed bystep 64, where epitaxial deposition of additional semiconductormaterials is performed as required.

FIGS. 8-10 show additional SIMS data taken on structures similar to thestructure 18 for four different thicknesses t₁ of the AlAs layer 22.FIG. 8 is a P profile showing how much the phosphorous diffusion in theGe substrate is reduced with a thickness of AlAs of only 1.4 Å. FIG. 9is an As profile showing very little diffusion of As into the Gesubstrate. FIG. 10 is a Ge profile showing that the presence of the AlAslayer significantly reduces the outdiffusion of Ge into the bottom partof the III/V layers. Ge typically is an n-type dopant in III/Vmaterials. Enhanced outdiffusion of Ge will prevent the placement of ap-n junction close to the nucleation layer. Each of FIGS. 8-10 show atrace of AlAs mass profile to identify the location of the interface forthe structure deposited with t₁=1.4 A to t₁=5.6 A. In the case ofstructure 18 deposited with t₁=0, there is obviously no Al detected atthe interface, but it's location within the semiconductor can beapproximated from the 72Ge or 31P profile. FIGS. 11-13 show the same setof data but this time analyzed against material standards which allowthe conversion of sputter time to profile depth and count rates toatomic concentrations (corrected for relative abundance of sampledisotopes). As in FIGS. 8-10 they show the effect of the AlAs layerthickness on the atomic concentration of P, Ge and As respectively as afunction of sample depth. A vertical stippled line marks the boundarybetween the III-V layer and the Ge substrate. FIG. 11 shows how the Pdiffusion into the Ge substrate is reduced with an increase in thicknessof the AlAs layer. FIG. 12 shows how the diffusion of As into the Gesubstrate can be tailored by choosing an appropriate thickness of theAlAs layer. FIG. 13 shows how the diffusion of Ge into the III-V layeris reduced with an increase in thickness of the AlAs layer. Onemonolayer of AlAs is sufficient to have the Ge atomic concentration dropto or even below 1×10¹⁷ cm⁻³ within 150 nm from the interface with theGe substrate.

FIG. 14 shows current plotted as a function of voltage for a Ge solarcell having a structure similar to 18. This Ge solar cell has anopen-circuit voltage (V_(oc)) of 0.247 mV, a resistance at V_(oc) of 7.2ohms, a short circuit current density (J_(sc)) of −36 mA/cm², a seriesresistance of 2 ohms and a fill factor of 60.5%, the fill factor being ameasure of the squareness of the current/voltage plot. These parametersare indicative of a Ge diode having a good performance.

FIG. 15 shows a series of plots of current as a function of voltage forGe solar cells fabricated with and without an AlAs nucleation layer. Twocurrent/voltage plots of solar cells without an AlAs nucleation areindicated by the arrow. For these cells, the V_(oc)=280 mV, J_(sc)=−36mA/cm², the series resistance is 2 ohms and the fill factor is 63%.Indicative also of a good diode performance in forward bias, but, asindicated by the arrow, the reverse breakdown voltage is very poor(approximately −0.2 V). The current/voltage plots of solar cells havingan AlAs nucleation layer are the ones not showing a breakdown voltage,demonstrating that the nucleation with the AlAs provides overallsuperior diode performance. Even more importantly is the smoothermorphology obtained in the case when the AlAs nucleation layer is used,as this will typically be critical for the performance of the otheractive elements to be grown above this p/n junction, as is typicallydone in, for example, solar cells.

Although the above exemplary embodiments show the growth of III/Vstructures on Ge substrates, a worker of ordinary skill in the art willreadily understand that other types of group IV substrates can be used.Similarly, although a binary AlAs compound was mentioned as a nucleatinglayer, it is to be understood that ternary or quaternary III/V compoundscontaining AlAs can also be used as nucleating layers without departingfrom the scope of this invention. As will be understood by a workerhaving ordinary skill in the art, the present invention is equallyapplicable to the fabrication of devices on all types of group IVsubstrates with or without the inclusion of a p-n junction. Further, aswill be understood by the skilled worker, other combinations of III-Vcompounds could be substituted to AlAs when there is a significantdifference in size, or electrochemical potential for surface binding,between the group III and the group V atoms. Such III-V compoundsinclude, for example, AlN, AlSb, or, BAs, BSb, GaN, GaSb, InN, or InAs.

As will be understood by a worker skilled in the art, although the abovedescription referred to p-type group IV substrates, other types of groupIV substrates can be used. Such substrates include n-type, undoped andsemi-insulating substrates.

The present invention provides a method for fabricating electronic oropto-electronic devices having a group IV substrate on which a III/Vlayer structure is deposited. The method allows for the manufacturing ofdevices with improved morphology and controlled doping profiles of groupV constituents into the group IV substrate and group IV constituentsinto the III/V layers. Devices fabricated according to the presentinvention have very good reverse breakdown voltage characteristics aswell as excellent forward bias characteristics, in addition to a smoothmorphology which is ideal for the epitaxy of additional active layersabove the p/n junction produced or not during the nucleation sequence.

The above-described embodiments of the present invention are intended tobe examples only. Those of skill in the art may effect alterations,modifications and variations to the particular embodiments withoutdeparting from the scope of the invention, which is defined solely bythe claims appended hereto.

1. A semiconductor device comprising: a group IV layer; and a nucleatinglayer formed on the group IV layer, the nucleating layer including aIII-V compound having at least aluminum (Al) as a group III element andat least one of arsenic (As), nitrogen (N) and antimony (Sb) as a groupV element.
 2. The device of claim 1 wherein, the III-V compound furtherincludes at least one of gallium (Ga) and indium (In) as a group IIIelement.
 3. The device of claim 1 further comprising a first III-Vcompound layer formed on the nucleating layer.
 4. The device of claim 3wherein, the first III/V compound layer includes at least one of GaInP,AlInP, and AlGaInP.
 5. The device of claim 3 further comprising a secondIII-V compound layer formed on the first III/V compound layer.
 6. Thedevice of claim 5 wherein, the second III-V compound layer includesGaAs.
 7. The device of claim 1 wherein, the group IV layer has a p-njunction proximate the nucleating layer.
 8. The device of claim 1wherein, the group IV layer is one of a p-type layer, an n-type layerand an undoped layer.
 9. The device of claim 1 wherein, the device is anelectronic device.
 10. The device of claim 9 wherein, the electronicdevice is an opto-electronic device.
 11. The device of claim 10 wherein,the opto-electronic device is a solar cell or a light emitting diode.12. The device of claim 1 wherein, the group IV layer is selected fromthe group consisting of a Ge layer, a Si layer, a SiGe and a SiC layer.13. The device of claim 1 wherein, the group IV layer is a group IVsubstrate.
 14. The device of claim 13 wherein, the group IV substrate isa vicinal substrate.
 15. The device of claim 14 wherein, the vicinalsubstrate has an angle ranging from 0° to 20°.
 16. The device of claim 5wherein, at least one of the nucleating layer, the first III/V compoundlayer and the second III/V compound layer is formed by an epitaxialgrowth process.
 17. The method of claim 1 wherein, a thickness of thenucleating layer ranges from 1 to 20 monolayers.
 18. A method offabricating a semiconductor structure on a group IV layer, the methodcomprising steps of: forming a nucleating layer on the group IV layer,the nucleating layer including a III-V compound having at least aluminum(Al) as a group III element and at least one of arsenic (As), nitrogen(N) and antimony (Sb) as a group V element; and forming a first III-Vcompound layer on the nucleating layer.
 19. The method of claim 17further comprising a step of forming a second III/V compound layerstructure on the first III/V compound layer.
 20. A method of controllingthe doping profile of a p-n junction formed in a group IV substrate, themethod comprising steps of: forming a nucleating layer on the group IVsubstrate, the nucleating layer including a III-V compound having atleast aluminum (Al) as a group III element and at least one of arsenic(As), nitrogen (N) and antimony (Sb) as a group V element; and forming aIII-V compound layer on the nucleating layer, the nucleating layer forcontrolling the diffusion of group V elements into the group IVsubstrate and for controlling the diffusion of group IV elements out ofthe group IV substrate.